1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming a full-color image by using the electrophotographic system, such as a copying machine, a printer, a facsimile apparatus, etc.
2. Description of Related Art
Among image forming apparatuses for forming images by electrophotographic processes, some of them are practicably arranged to be capable of forming images in full color. In order to form a full-color image at a high speed, it has been known to adopt a so-called tandem-type arrangement whereby a plurality of image forming parts (image forming units) are arranged in the direction of transporting a recording material.
In forming color images, causes of deteriorating image quality include positional discrepancy of component color images (hereinafter referred to as the color position discrepancy). The color position discrepancy takes place in cases where the positions of various component color images which constitute a full-color image deviate from each other in the direction of auxiliary scanning or main scanning or where they fail to be in parallel with each other.
In the case of the above-stated tandem-type image forming arrangement, images in different colors are formed at the respective different places. The tandem-type image forming apparatus is, therefore, more prone to the color position discrepancy than the conventional apparatus having only one image forming part (one photosensitive drum).
The color position discrepancy takes place in varied directions. The color position discrepancy taking place in the direction of auxiliary scanning results from static causes and dynamic causes. The static causes include deviation mainly caused by errors in respect of assembly or machining precision of parts, such as deviation from a correct distance between one image forming unit and another image forming unit, i.e., a difference in distance between photosensitive drums or exposure positions, and the precision of diameter or the like of a driving roller arranged to drive a belt-shaped recording-material bearing member which transports or conveys a recording material at a controlled speed at the time of transfer (for example, a belt member such as a transfer belt). The dynamic causes include fluctuations of the rotating speed of the photosensitive drum or the transfer belt, etc.
The static causes are removable by a correction process, for example, by electrically adjusting exposure timing, a least at the time of shipping the apparatus from a manufacturing factory.
The dynamic causes are, on the other hand, difficult to eliminate by any correction process. The fluctuations of the rotating speed of the photosensitive drum and the fluctuations of the transport speed of the recording material by the transfer belt, however, must be minimized. To attain this purpose, therefore, efforts have been exerted in various manners to improve the precision of a drive source such as the above-stated driving roller, etc., and a method of control over the drive source.
For example, the apparatus is arranged to prevent any eccentricity of a driving roller from contributing to the color position discrepancy by arranging the distance between the image forming units to be integer times as much as the circumference of a driving pitch circle defined by the neutral plane of the transfer belt.
However, in a case where a belt member is used, the eccentricity of the driving roller is only one of causes for fluctuations of the speed. For example, to transmit a rotative driving force to the transfer belt without any slip, the driving roller is provided with a rubber layer on its surface. Therefore, the use of the driving roller over a long period of time causes some wear of its surface or some peripheral wear of the belt, which causes some change in radius from the center of the driving roller to the neutral plane of the belt, and thus eventually causes a change in linear speed of the belt.
Even a slight degree of such a wear brings about the color position discrepancy. For example, with the diameter of the belt driving roller assumed to be D (mm), the thickness of the belt to be T (mm) and an image forming speed to be V (mm), the diameter of the neutral plane of the belt (the diameter of the pitch circle) is xe2x80x9cD+Txe2x80x9d (mm). With N assumed to be an integer, the distance between the image forming units is represented as xe2x80x9cNxc3x97xcfx80xc3x97(D+T)xe2x80x9d. In a case where the apparatus is made in the smallest size, therefore, the distance between the image forming units becomes xe2x80x9cxcfx80xc3x97(D+T)xe2x80x9d (mm).
Assuming that the amount of decrease in thickness of the belt is expressed as xcex94T and the amount of decrease in diameter of the driving roller as xcex94D, the amount of change of the image forming speed can be expressed as follows:
(xcex94T+xcex94D)/(T+D)xc3x97V(mm/sec)xe2x80x83xe2x80x83(1)
Generally, a full-color image is formed by using four image forming units. Therefore, a distance between the furthest-parted image forming units is xe2x80x9c3xc3x97xcfx80xc3x97(T+D)xe2x80x9d (mm). A period of time necessary for passing these image forming units at a normal image forming sped then can be expressed as follows:
3xc3x97xcfx80xc3x97(T+D)/V(sec)xe2x80x83xe2x80x83(2)
Therefore, the amount of position discrepancy between component color images taking place between the furthest-parted image forming units can be obtained by multiplying the formulas (1) and (2) by each other as follows:
3xc3x97xcfx80xc3x97(xcex94T+xcex94D)xe2x80x83xe2x80x83(3)
In other words, even in a case where the diameter of the roller is worn and decreased by only 5 xcexcm and the thickness of the belt also by only 5 xcexcm, for example, the amount of color position discrepancy reaches about 94 xcexcm, as found from the formula (3), so that the color position discrepancy which exceeds two pixels in the case of resolution of 600 dpi would occur.
Various methods have been developed against such a color position discrepancy due to the wear of parts or due to other disturbances or causes. Known prior art methods for this purpose include the following: (i) Means for reading an image recorded on the belt member is provided, and exposure timing or an exposure position is controlled on the basis of the result of reading. (ii) The moving speed of the belt member is detected, as desired, from a pattern formed on the belt member, and the speed of the belt member during an image forming process, exposure timing or an exposure position is controlled, as necessary, on the basis of the result of detection.
However, according to the method (i), the image forming apparatus must be arranged to include means for reading the formed image and the mechanism for correcting the exposure positions. This method inevitably causes an increases in cost, necessitates complex arrangement and an increase in size of the apparatus.
The method (11) necessitates a high-resolution encoder to be arranged on the belt member and requires control over the transport speed of the belt member and the exposure positions during the image forming process. This method, therefore, has the same shortcomings as those of the method (i).
It is an object of the invention to provide an image forming apparatus simply arranged, without recourse to any complex arrangement, to be capable of forming an image without any color position discrepancy despite of occurrence of changes caused by aging in the thickness of a belt member and in the diameter of a driving roller which is arranged to drive the belt member.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.